1. Field of the Invention
The present invention relates to a device and method of improving picture quality of a television set or video projector.
2. Description of the Related Art
The occurrence of flare is known as a cause for degradation of picture quality in an image display device such as a television set or video projector. Flare is a phenomenon in which light in bright areas infiltrates dark areas due to reflection or scattering of light on the lens or projection surface of a picture tube or projector, thereby causing fading at edges (for example, at the border between a white area and a dark area) in a displayed image in which the difference in luminance is great.
The prior art is described based on the description in the specification of Japanese Patent Application No. 2001-086516, which is in an invention that is related to the present invention.
FIG. 1 is a schematic figure showing an example of the original image of an image that is projected by a projector. This original image has a rectangular white area WT in the center that is surrounded by a black area BL, the edge portion ED at the boundary of these two areas having a large difference in luminance. The lower portion of FIG. 1 shows the horizontal video signal (luminance signal) in the vicinity of the center of the original image. When this original image is projected onto a screen by a projector, light of the white area WT infiltrates the black area BL and degrades the edge portion ED, thereby giving rise to flare and reducing the picture quality.
To eliminate the above-described flare, the video signal that is applied as input to the projector is typically subjected to digital signal processing to correct degradation of the edge portions. FIG. 2 is a schematic figure of flare correction, (a) showing waveform chart showing the video signal of the original image, (b) showing the luminance distribution of the screen image that is displayed by means of the video signal of (a), (c) showing a waveform chart of the signal after flare correction of the video signal of (a), and (d) showing the luminance distribution of the screen image that is displayed by means of the video signal after flare correction of (c). Here, the video signal of FIG. 2(a) corresponds to the video signal of the original image that is shown in the above-described FIG. 1.
The screen image that is obtained by using a projector to project the video signal of FIG. 2(a) is an image having edge portions as shown in FIG. 2(b) that have been blunted by the occurrence of flare. To correct this flare, correction (reverse correction) as shown in FIG. 2(c) that corresponds to the blunting of the edge portions shown in FIG. 2(b), i.e., correction that sharpens the edges, should be applied at the rises and falls in the video signal of FIG. 2(a). This correction allows a screen image to be obtained that has no blunting of the edge portions, such as shown in FIG. 2(d).
FIG. 3 shows an example of a picture quality improvement device that performs the above-described flare correction. Of a luminance signal and a color signal (broadband color signal and narrowband color signal) obtained from the primary color signals R-G-B, the picture quality improvement device generates a correction signal for correcting flare from the luminance signal and adds this flare correction signal to the original luminance signal to correct flare. This picture quality improvement device is made up by: vertical low-pass filter (VLPF) 71, horizontal low-pass filter (HLPF) 72, delay circuit 73, subtracter 74, gain adjustment circuit 75, and adder 76.
VLPF 71 takes as input luminance signal YIN that is obtained from primary color signals R-G-B, and has the two outputs “Main out,” for outputting the received luminance signal YIN without modification, and “LPF out” for outputting the vertical low-pass component of the received luminance signal YIN that has been extracted. “Main out” is supplied to delay circuit 73, and “LPF out” is supplied to HLPF 72.
Delay circuit 73 applies a time delay that exactly corresponds to the time required for filtering the received signal in VLPF 71 and HLPF 72, and the output of delay circuit 73 is applied to each of the “+” input of subtracter 74, and one of the inputs of gain adjustment circuit 75 and adder 76.
HLPF 72 extracts the horizontal low-pass component from the signal of “LPF out” of VLPF 71, i.e., the signal in which the vertical low pass component of luminance signal YIN has been extracted. VLPF 71 and HLPF 72 together constitute a two-dimensional low-pass filter for flare correction. The output of HLPF 72 (low-pass extraction signal) is supplied to the “−” input of subtracter 74. VLPF 71 and HLPF 72 are both constituted by a FIR (Finite Impulse Response) filter or an IIR (Infinite Impulse Response) filter that is composed of a plurality of registers. For example, VLPF 71 or HLPF 72 may be constituted by the combination of a 12-Tap FIR filter and a 1-Tap IRR filter.
Subtracter 74 subtracts the low-pass extraction signal (the output of a two-dimensional low-pass filter) that is supplied to the “−” input from HLPF 72 from the luminance signal (this being the original luminance signal) that is supplied to the “+” input from delay circuit 73 and supplies the edge signal, which is the subtraction result, to gain adjustment circuit 75.
Gain adjustment circuit 75 prevents a decrease in the sensitivity of the flare correction filter (in this case, a two-dimensional low-pass filter composed of VLPF 71 and HLPF 72) in dark portions of an image in which the signal level is low when a nonlinear signal that is multiplied by gamma is taken as the input signal. Normally, a gamma-multiplied nonlinear signal is taken as the input signal in an image display device such as a television set or projector in consideration of the characteristics of the construction (for example, the characteristic of the cathode ray tube). When such a signal is subjected to filter processing to produce a correction signal (flare correction signal) and added, the linearity of the correction signal itself is lost, the sensitivity of the correction filter falls in dark areas of an image in which the signal level is low, and sufficient improvement in the picture quality is not obtained in dark areas. To prevent this state, the gain in correction signal that is produced by the flare correction filter is regulated by gain adjustment circuit 75. The output of this gain adjustment circuit 75, i.e., an edge signal that is corrected such that the gamma characteristic is linear, is supplied to the other input of adder 76.
Adder 76 adds the edge signal that has been subjected to gain regulation by gain adjustment circuit 75 to the luminance signal (original luminance signal) that is supplied from delay circuit 73 and outputs luminance signal Yout, which is the result of this addition.
In a picture quality improvement device that is constructed according to the foregoing explanation, luminance signal YIN is applied as input to VLPF 71 where the vertical low-pass component is extracted, then applied as input to HLPF 72, where the horizontal low-pass component extracted. At the same time, luminance signal YIN is supplied to delay circuit 73 by way of VLPF 71 and subjected to a prescribed delay by this delay circuit 73.
A low-pass extraction signal in which the low-pass components in each of the vertical and horizontal directions have been extracted by VLPF 71 and HLPF 72 is supplied to the “−” input of subtracter 74, and the original luminance signal that has undergone a prescribed delay by delay circuit 73 is simultaneously supplied to the “+” input of subtracter 74. In subtracter 74, the low-pass extraction signal that is supplied to the “−” input is subtracted from the original luminance signal that is supplied to the “+” input to obtain an edge signal.
FIG. 4(a) is a waveform chart of the edge signal, and FIG. 4(b) is a waveform chart of the original video signal. The edge signal that is shown in FIG. 4(a) is equivalent to edge portions ED relating to the horizontal direction of the original image that is shown in the previously described FIG. 1; the original video signal shown in FIG. 4(b) is equivalent to a video signal of the original image shown in FIG. 1; and adding this edge signal and original video signal can obtain a signal waveform following flare correction that was shown in the previously described FIG. 2(c). In the present form, subtracting the low-pass extraction signal in which the low-pass components in the vertical and horizontal directions have been extracted by means of VLPF 71 and HLPF 72 from the original luminance signal that has undergone a prescribed delay by means of delay circuit 73 obtains the edge signal that is shown in FIG. 4(a).
After undergoing correction by means of gain adjustment circuit 75 such that the gamma characteristic becomes linear, the edge signal that is outputted from subtracter 74 is added by means of adder 76 to the original luminance signal that has been supplied from delay circuit 73, whereby the signal waveform after flare correction that is shown in the previously described FIG. 2(c) is obtained. The output of this adder 76, which is luminance signal Yout, and a color signal (broadband color signal and narrowband color signal), which is obtained from the primary color signals R-G-B, are applied as input to a known matrix circuit (not shown in the figure) to reconvert to an R signal, a G signal, and a B signal, which are the primary color signals. An image is displayed based on the R signal, G signal, and B signal that have been reconverted in this way.
To apply the picture quality improvement device that is constituted as described hereinabove to wide-screen (the aspect ratio of the screen being 16:9), the following process is carried out. A number of display modes exist for applying an image in which the screen aspect ratio is 4:3 to a wide screen in which the aspect ratio is 16:9. FIG. 5 is a figure for illustrating display modes when dealing with a wide screen, (a) showing display on a wide-screen of an original image in which the screen has an aspect ratio of 4:3, (b) showing the display in normal display mode, (c) showing display in full display mode, (d) showing display in zoom display mode, and (e) showing display in nonlinear display mode.
As shown in FIG. 5, when an image in which a circle is drawn in the center for a screen having an aspect ratio of 4:3 is displayed on a wide screen in which the screen has an aspect ratio of 16:9, four display modes exist: normal display mode, full display mode, zoom display mode, and nonlinear display mode.
In normal display mode, the original image of FIG. 5(a) is displayed without alteration in the center of the screen as shown in FIG. 5(b) (this display area being the effective area) and both sides are black areas. In full display mode, an image is displayed in which the original image of FIG. 5(a) is enlarged horizontally at a prescribed magnification as shown in FIG. 5(c). In zoom display mode, an image is displayed in which the original image of FIG. 5(a) is enlarged both vertically and horizontally at the same magnification as shown in FIG. 5(d). In nonlinear display mode, an image is displayed in which the original image of FIG. 5(a) has been subjected to nonlinear image processing in which proportions are equal to the original image within a prescribed region in the center but in which magnification increases with greater distance from the center.
When dealing with a wide screen, display is normally set to any one of the above-described display modes, but a device having the above-described construction entails the following processing when setting to display modes.
(1) Normal Display Mode
When set to normal display mode, an image having black areas on both sides is processed as the original image (an original image having an aspect ratio of 4:3 originally has black areas (blanking areas) above and below and to the left and right of the image), for example, as shown in FIG. 5(b). The device shown in FIG. 3 is constituted for detecting edges, and the edges at the boundaries of the black areas and the effective area therefore rise when processing such an original image, these edges thereby constituting a cause of degradation of picture quality. To prevent this degradation, the following process is carried out.
FIG. 6 is a waveform chart showing horizontal video signal in the normal display mode, a1 being the black area (blanking area) to the left of the image, b being the effective area, and a2 being the black area (blanking area) to the right of the image. In HLPF 72 of the circuit that was previously described in FIG. 3, values Ps of the start of the image are set in all registers at the start timing of effective area b, the values of the registers are shifted to update the image during the interval of effective area b, and the end values Pe are set (held) in the registers without updating the image in black area a2.
Blanking areas normally exist in the upper and lower directions of the screen, and a process that is similar to the above-described processing in HLPF 72 is therefore also performed in VLPF 71.
(2) Full Display Mode
When set to full display mode, correcting flare by means of the circuit shown in FIG. 3 and then extending the original image horizontally at a prescribed magnification means that the correction values themselves are also extended, whereby a sufficient correction effect, which is the object, cannot be obtained. In such a case, the response characteristic of HLPF 72 must be altered (shortened) in accordance with the magnification at which the image is extended. Specifically, the impulse response of HLPF 72 (the filter coefficient) is set such that the edge width of the edge signal, which is the output of subtracter 74, is reduced to the degree that the image is extended.
(3) Zoom Display Mode
When set to zoom display mode, the effect upon correction of extension of an image when in the above-described full display mode also occurs in the vertical direction. In this case, the response characteristic of VLPF 71 and HLPF 72 must be changed (made shorter) according to the magnification of extension of the image. Specifically, the impulse responses (filter coefficients) of VLPF 71 and HLPF 72 are set such that the edge width of the edge signal, which is the output of subtracter 74, is reduced to the degree that the image is extended.
(4) Nonlinear Display Mode
When set to nonlinear display mode, correction is carried out by means of the circuit shown in FIG. 3, following which the original image is subjected to nonlinear extension, whereby the correction value itself undergoes extension and the object correction effect cannot be obtained. In this case, a process for applying nonlinear enlargement to the original image must be performed before the correction by the circuit shown in FIG. 3. In this case, the nonlinear processing circuit for realizing the nonlinear display mode may be provided on the input side of the circuit shown in FIG. 3.
When an image that is displayed by means of video signals is applied to a wide screen by enlarging at a prescribed magnification in the horizontal direction, the vertical direction, or both directions, the amount of amplitude (the size of “a” in FIG. 4) of the edge signal shown in FIG. 4(a) must be adjusted according to the enlargement magnification. In order to realize this adjustment of the amount of amplitude of the edge signal in the picture quality improvement device of the prior art, the vertical low-pass filter and horizontal low-pass filter are configured such that the response characteristics of each are shorter, to a degree that is equal to the above-described prescribed magnification, than the target response characteristic when there is no enlargement of the image. Specifically, the set values of the impulse response (filter coefficients) of the vertical low-pass filter and horizontal low-pass filter are modified. Normally, to realize a filter in which the filter coefficient can be modified, a multiplier is required in the coefficient unit that constitutes the filter, whereby the filter circuit configuration inevitably becomes large. Consequently, there is the problem that a picture quality improvement device of the prior art entails large circuit scale because it requires low-pass filters in which filter coefficients can be freely set.